Лови
<Border width = "200" Height ="30" HorizontalAlignement="Left" VerticalAlign="Top"
BorderBrush="Green" BorderThickness="1" CornerRadius="15" Margin="50,100,0,0"
<Border Name="prog" Width="0" Height="30" BorderThickness="0" CornerRadius="15"
HorizontalAlignment="Center" VerticalAlignment="Center">
<Border.Background>
<LinearGradientBrush StartPoint="0,0" EndPoint="1,1">
<GradientStop Color="White" Ofset="0,0"></GradientStop>
<GradientStop Color="Blue" Ofset="0,4"></GradientStop>
</LinearGradientBrush>
<Border.Background>
</Border>
</Border>